Mass spectroscope having means for compensating for electron space charge in the ion source



y 3. 1969 c. BRUNNE'E ET AL 3,444,366

MASS SPECTROSCOPE HAVING MEANS FOR COMPENSATING FOR ELECTRON SPACE CHARGE IN THE ION SOURCE Filed Sept. 3, 1965 Sheet of 2 Fig.1

Fig.2

1N VENTOR.

Q85; 8mm i QWW May 13, c BRUNNEE ET AL MASS SPECTROSCOPE HAVING MEANS FOR COMPENSATING FOR ELECTRON SPACE CHARGE IN THE ION SOURCE Filed Sept. 5, 1965 Sheet 2 of 2 Fig.3 if I '200 Ug2600l "4/: l lol 510- 0 Fig.4 5 ME, 176/5 Uz %/I 4 NEQ/ M528 T 3 M540 IN VENTOR United States Patent US. Cl. 250-413 1 Claim ABSTRACT OF THE DISCLOSURE A mass spectroscope in which the ion acceleration voltage is altered to effect scanning has the ion extraction voltage formed by superimposing a voltage Which varies proportionally to the acceleration voltage upon a supplementary voltage which is independent of the acceleration voltage so that the influence of the electron space charge on the ion paths and thus upon the sensitivity is compensated.

The invention relates to mass spectroscopes in which the mass-dependent deflection in a magnetic field is caused by variation of the acceleration voltage. The problem underlying the invention is the compensation of the space charge elfect of the electrons in these devices. Compensation of the space charge effect of the electrons is not necessary in devices in which mass scanning is ef fected by alteration of the magnetic flux of separating magnets. In these devices, the electron space charge has simply the effect that on variation of the electron current the voltages for ion focusing have to be adjusted. This is different in the case of devices having mass scanning by variation of the ion acceleration voltage.

The electron space charge alters the electric field produced in the ionisation chamber by the draw-out voltage U so strongly, that with the alteration of the acceleration voltage necessary for electrically scanning the masses the focusing of the ions is disturbed if the voltage lying on the electrodes of the ion acceleration path, to which also the draw-out voltage belongs, are varied proportionately. In order to avoid these disturbances of focusing, which arise in these devices, if the draw-out voltage U is altered proportionally to the acceleration voltage, as in the space-charge-free case, according to the invention, on scanning the masses by means of voltage alteration, the draw-out voltage is not varied proportionally to the acceleration voltage. Instead the draw-out voltage is varied according to another function which can be determined empirically and which is so selected that the influence of the electron space charge on the ion paths and thereby the partial-pressure sensitivity is compensated. It can thus be achieved that when scanning the masses, the focusing of the ions remains undisturbed. Preferably, the draw-out voltage U is formed by superimposing a voltage which follows proportionally to the acceleration voltage, with a supplementary voltage which is independent of the acceleration voltage, the supplementary voltage advantageously being so selected that it is dependent on the electron current or the space charge caused thereby.

The above and other objects and advantages of the invention, will be clear from the following description, taken with reference to the accompanying drawings which are given by way of example and in which:

FIG. 1 is a diagrammatic view of a mass spectroscope of the invention;

FIG. 2 is a diagram of the potential distributions in i "ice FIG. 4 is a graph showing the draw-out voltage U in relation to the acceleration voltage U The mass spectroscope illustrated operates in a known manner with electric mass scanning. It comprises a vac uum container 1 with an ion source 2, into which the substance to be analyzed is introduced in gaseous or vapour form, e.g. from a storage container 3.

The ion source consists of a metal box 4 through which an electron beam A for impact ionisation passes between electrodes 5a, 5b. A magnet 6 serves for focusing the electron beam, which in a manner similar to the electron beam A with the electrodes 5a, 5b, is in reality displaced through 90 relative to the plane of the drawing, so that its field is in register with the stray field of the separation magnet in the region of the electron beam A.

The ions formed in the ionisation chamber 7 are drawn from the box 4 by a draw-out voltage U on an electrode 8, through a window 9 in this electrode, and are guided through further electrodes 10 and 11 serving for accelerating and focusing, having lens voltages U and U so that focusing of the ion beam B takes place on the entry gap 12 in the sector field of the magnet 13. The focusing takes place mainly in the X-direction. In order to achieve a certain bundling or focusing in the Z-direction which is perpendicular to the plane of the drawing, the electrodes 11b are provided.

In the sector field of the magnet 13, the ions of the ion beam B are deflected. The amount of this deflection depends on the strength of the magnetic field, the ion mass and the ion acceleration voltage between the metal box 4 and the exit electrode 11. With constant magnetic field and constant acceleration voltage U the deflection is determined solely by the mass number of the ions. For separately intercepting the ions according to their mass number an exit electrode 14 with an exit gap 16 and an electrode 16 serving as an interceptor is provided, which is connected to an arrangement 17 for measurement of the intercepted ion current. By alteration of the acceleration voltage U all ions in the sector field, with their different mass numbers, are successively brought onto the interceptor 16 and the measurement is thereby effected qualitatively and quantitatively in the arrangement 17.

For the ion source, the following voltage supply is provided.

Draw-out voltage U on the electrode 8: constant voltage U adjustable between 0 and 3 volts, together with a co-varying voltage, adjustable between 0 and 8.10 U (U =24 volts for U :3 kv.).

Lens voltage U on the lens 10: average value fixedly set to about U =8.l0 'U (U =24 volts for U =3 kv.).

For an interceptor with r=6.5 the deflection voltage is adjustable between 25.10- -U (U ,=O volts for U =3 kv. and 36.10 -U (U =108() volts for U =3 kv.).

The voltage supply is so arranged that the ion space charge, electron space charge, the magnetic field provided for focusing the electron beam and the Z-focusing can be allowed for in a favourable manner for the measurement.

The following explanation will enable the relationships to be understood:

In accordance with geometric ion-optics, without allowing for the space charge, the paths of individual charged particles, which have the energy 0 ev. at the place of origin and which are accelerated, deflected and focused by electric lens systems, depend only on the relationships of the voltages lying on the relevant lens electrodes. Thus, if all voltages are proportionally varied, then the ion paths remain the same and with the absence of a magnetic field are independent of the mass. Thus, if all voltages in an ion source are proportionally varied, as is usuv 3 ally effected in electrically scanning the masses, then the ion paths in the ion source and thus the sensitivity remain the same. This is however not the case in practice. The reason for this is that the requirements which lead to the simple law of imagery, are not satisfied by an ion source. In the ionisation chamber, space charges are present, originating from ionising electrons and also from the resultant ions. Furthermore, for focusing the electrons, a magnetic field, namely the field of the magnet 13 is present. The energy of the resultant ions is not exactly zero, firstly because many ions receive a certain initial energy of the order of 1 ev. during the ionisation process (formation of molecule fragments), and secondly because the place of origin of the ions in the ionisation chamber does not exactly lie on an equipotential surface, i.e. the ions result at regions of somewhat different voltages due to the presence of a draw-out voltage. These four influences: space charge of the electrons, space charge of the ions, magnetic field and initial energy of the ions, influence the ion-optical image formation and have to be specially allowed for in construction of the ion source and in the voltage supply, if the scanning of masses is performed by alternation of the acceleration voltage U The influence of the space charge can be simulated if the change in the ion paths is analyzed, with variation of the draw-out voltage U The ion-optics must be so arranged that the focusing of the ions, and thus the sensitivity, changes as little as possible with alteration of the draw-out voltage.

The electron space charge, as explained above, has a much stronger influence than the ion space charge in the present mass spectrometer with electric mass scanning.

For the case of a plane parallel field the potential which is effective in the ionisation chamber 7 (precise calculation according to Brubaker, Influence of Space Charge on the Potential Distribution in Mass Spectrometer Ion Sources, 26 Journal of Applied Physics 1007 [August 1955]) is given by Where is the part originating from the draw-out voltage U and go, is the part originating from the space charge of the electrons, i.e. from the electron current i x indicates the coordinates in the travelling direction of the ions- Psp is given by (see FIG. 2)

where d is the depth of the ionisation chamber. can be approximately represented by eg or a similar expression U (i,,)f(x). If to a coarse approximation it is assumed that the ion paths are essentially determined by the effective field strength (P e1f(xm) at a central region x between the electron beam and the drawout electrode (inclination of the potential curve, FIG. 2), then if the space charge is to be allowed for, as mentioned above, on scanning by means of voltage variation, this effective field strength will vary proportionally to the accele-ration voltage, i.e. it must be B= z +f'( m) UIUB) where k is a constant of proportionality.

This equation means that the draw-out voltage U must be composed of a factor kU proportional to the acceleration voltage and a factor f'(x U (i which is independent of the acceleration voltage.

The measurements which are effected give the following indication:

With alteration of the draw-out voltage in proportion to alteration of the acceleration voltage, the operating point is considerably displaced, i.e. if the ion current is entered in a graph in dependence on the draw-out voltage expressed as a percentage of the acceleration voltage, then various characteristics (FIG. 3) are resulting. When scanning the masses, the characteristic is to be followed. If the operating point is at a maximum for one mass (pressure proportional) then this is no longer so for another mass (not pressure proportional). If according to Equation 4 the draw-out voltage component proportional to the acceleration voltage has superimposed thereon a fixed DC. voltage U then the effect can be well compensated, as shown by FIG. 4, in which the draw-out voltage U =C U +U is shown and the respective positions of the characteristic peaks for a number of masses. U and C have to be empirically obtained when adjusting the ion source, in order that the straight line joins as well as possible the draw-out voltage range given by the characteristic peaks. In practice, adjustment has to be made only for two spaced apart masses, which can be performed in a few minutes. The adjustment is not critical, owing to the broad characteristic peaks. Thus, if the draw-out voltage is altered in accordance with the function C U +U then with scanning the masses the position of the operating point on the characteristic remains constant. The DC. voltage U to be fed in is of the order of 1 volt at 60 a. electron current. As can be expected from the above theoretical considerations, the voltage increases with increasing electron current. As an approximation, the empirically obtained formula U vpta. 0,1[V] is applicable.

With very low electron currents the straight line passing through the characteristic peaks, as has been measured, actually passes through the zero point, i.e. with negligibly small electron space charge U =C U in accordance with the theory for the space-charge-free case. From this it follows that the ion space charge on scanning the masses is of essentially less importance than the electron space charge.

We claim:

1. In aspectroscope including a source of an ion extraction voltage in which the scanning of the masses is effected by alteration of the ion acceleration voltage, the improvement comprising means for varying said ion extraction voltage in dependence on the acceleration voltage in accordance with a function by which the influence of the electron space charge on the ion paths and thus on the sensitivity is compensated,

wherein said source of an ion extraction voltage includes means for superimposing a voltage which varies proportionally to the acceleration voltage upon a supplementary voltage which is independent of the acceleration voltage,

said supplementary voltage being dependent on the electron current associated with said spectroscope producing said electron space charge.

OTHER REFERENCES Journal of Applied Physics, vol. 26, No. 8, Brubaker, August 1955, pp. 1007 to 1012.

RALPH G. NILSON, Primary Examiner. A. L. BIRCH, Assistant Examiner. 

